1. Field of the Invention
This invention relates generally to an optical device which makes use of a magneto-optical effect, and more particularly to an optical device for providing an arbitrary Faraday rotation angle or an arbitrary attenuation to an optical signal of each channel in wavelength division multiplexing (WDM).
2. Description of the Related Art
When a light beam passes through magneto-optical crystal of YIG (yttrium-iron-garnet) or the like placed in a magnetic field, a Faraday rotation angle is provided to the light beam by a magneto-optical effect in accordance with the magnitude and the direction of a magnetization vector of the magneto-optical crystal and the thickness of the magneto-optical crystal. An optical device which operates in accordance with this principle is called Faraday rotator, and Faraday rotators wherein a magnetic field is applied to magneto-optical crystal using a permanent magnet have been put into practical use. Since magnetization of magneto-optical crystal by a permanent magnet is uniform, the Faraday rotation angle is usually invariable in the Faraday rotator.
A variable optical attenuator is provided by combination of a Faraday rotator and a polarizer. For example, Japanese Patent Laid-Open Application No. Heisei 1-204021 discloses a variable optical attenuator wherein a magnetic field is applied to magneto-optical crystal using only one electromagnet. However, where only one electromagnet is used, magnetization of the magneto-optical crystal may not always be in a saturated condition. If magnetization of magneto-optical crystal is not saturated, then a large number of magnetic domains appear in the magneto-optical crystal. The presence of such a large number of magnetic domains deteriorates the reproducibility of attenuation of an optical attenuator or, even if good reproducibility is secured, makes continuous variation of the attenuation difficult. Further, scattering of light at interfaces between a large number of magnetic domains causes attenuation which is difficult to control.
An optical device wherein an electromagnet and a permanent magnet are used in combination to allow variation of a Faraday rotation angle while magnetization of magneto-optical crystal is saturated has been proposed by the inventor of the present invention (Fukushima et al., OAA, FD9 pp.154-157, 1996). This optical device is a variable optical attenuator and achieves a characteristic of attenuation which continuously varies from 1.6 dB to 25 dB by varying the driving current from 0 mA to 40 mA.
In recent years, a technique of production and a technique of use of an optical fiber of a low loss (for example, 0.2 dB/km) have been established, and optical communication systems wherein an optical fiber is used as a transmission line have been put into practical use. Further, in order to compensate for the loss of an optical fiber to allow long-haul transmission, use of an optical amplifier for amplifying signal light has been proposed or such optical amplifiers have been put into practical use.
One of conventionally known optical amplifiers includes an optical amplification medium to which signal light to be amplified is supplied, and means for pumping the optical amplification medium so that the optical amplification medium may provide a gain band including the wavelength of the signal light. For example, an erbium-doped fiber amplifier (EDFA) includes an erbium-doped fiber (EDF) serving as an optical amplification medium, and a pump light source for supplying pump light having a wavelength determined in advance to the EDF. By setting the wavelength of the pump light to the 0.98 .mu.m band or the 1.48 .mu.m band, a gain band including a wavelength of 1.55 .mu.m is obtained. Also another optical amplifier which includes a semiconductor chip as an optical amplification medium is known. In this instance, pumping takes place when current is injected to the semiconductor chip.
Meanwhile, as a technique for increasing the transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is available. In a system to which the WDM is applied, a plurality of optical carriers having different wavelengths from each other are used. A plurality of optical signals obtained by modulating the optical carriers independently of each other are wavelength division multiplexed by an optical multiplexer, and resulting WDM signal light is forwarded into an optical fiber transmission line. On the reception side, the WDM signal light received is demultiplexed into individual optical signals by an optical demultiplexer, and transmission data are reproduced based on the optical signals. Accordingly, by applying WDM, the transmission capacity of one optical fiber can be increased in accordance with the multiplexing number of optical signals.
Where an optical amplifier is incorporated in a system to which WDM is applied, the transmission distance is limited by a wavelength dependency of a gain represented by a gain tilt or a gain deviation. For example, with regard to an EDFA, it is known that a gain tilt appears in the proximity of the wavelength of 1.55 .mu.m and this gain tilt varies in accordance with the input power of WDM signal light and the power of pump light to the EDFA, the temperature of the EDFA and so forth.
In order to suppress the wavelength dependency of the gain, it may be proposed to first attenuate optical signals of different channels using an optical attenuator having a suitable attenuation and then perform wavelength division multiplexing of the optical signals. In this instance, while an arbitrary attenuation can be provided to each of the optical signals of the channels by using a plurality of optical attenuators, a number of optical attenuators equal to the number of channels of WDM are required, and consequently, a complicated construction or a large size apparatus is required.